The present invention relates to a rock anchor which utilizes a synthetic resin, especially an epoxy resin, which is cold injected in liquid form under pressure and hardens thereafter.
It is already known to stabilize rock formations by injecting hardenable plastic materials such as a polyester or an epoxy resin in liquid form under pressure and then letting these plastic materials harden. For the injection, an extruder head is placed on the external face of the rock, sealing it and penetrating into the bore hole to be filled. The injected resin in liquid form penetrates into the fissures adjacent to the hole to be filled, so that a "compound" with the rock is achieved (see U.S. Pat. No. 3,260,053).
The disadvantage of such a synthetic resin dowel is that it cannot be tensioned like reinforced concrete anchors. Reinforced concrete anchors, as is already known, consist of a steel rod or pipe introduced into a bore hole and bonded to the bore hole wall with a concrete material which fills the bore hole and becomes solidified. The outward projecting portion of the steel rod or pipe is provided with a thread for screwing a clamp nut thereto, said clamp nut pressing an anchor plate against the rock so that through the screwing of the nut the required tensional force can be generated.
Accordingly, the object of the present invention is to provide a rock anchor such as that described, which can be tensioned.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Pursuant to the present invention, the disadvantages of the prior art are solved by providing a rock anchor comprising a metallic tubular element, the extremity of which is threaded for receiving a nut, providing an anchor plate which is pressed against the rock, and providing as a reinforcing element at least one glass fiber-reinforced plastic rod, one end of which is maintained in the metallic tubular element by the addition of a hardened resin thereto with the use of a filler-inlet hole, with the other end extending into the bore hole to a certain depth. In addition, at least one sealing ring is disposed around the metallic tubular element directly at the threaded section, thereby sealing the bore hole wall during the introduction of the anchor head into the bore hole. For practical purposes, the metallic tubular element comprises deformations of its wall protruding beyond the inner surface thereof in order to increase the static friction at the filled-up resin.
The reinforcing elements used for this purpose can be glass fiber-reinforced plastic rods and tubes. The plastic material may be polyester resin or epoxy resin. The glass fiber-reinforced resin rods and tubes are manufactured by the following process. An endless wound glass fiber is cut to portions of the desired length of for instance a glass fiber-reinforced resin rod. The glass fiber portions are grouped to a pack of the desired thickness, the pack is impregnated with an expoxy or polyester resin, and then the pack is pressed or drawn through a correspondingly annular die. After the curing of the pack a glass fiber-reinforced resin rod or tube, respectively, is obtained. In this rod or tube the glass fiber material is predominant, because the epoxy resin or polyester resin merely serves the purpose to bond the glass fiber portions in the pack to each other.
The tube can be provided with air-relief slots at its end situated in the bore hole. As this tube is open at its outward end of the bore hole, the air which has been displaced by the introduction of the liquid resin can escape through this tube which also serves as a reinforcing element. Mountain water which might eventually accumulate inside the bore hole can also flow out through this tube. At the end of the injection process, the tube is filled with liquid resin, and the outflow of the resin from its end situated outside the bore hole indicates that the bore hole is completely filled up.
Besides the glass fiber-reinforced plastic tube, other reinforcing elements can be used, such as glass fiber-reinforced plastic rods. The material of the plastics used for the tube or rods may be a synthetic resin. They are placed and maintained in the metallic tubular element by the hardened synthetic resin, for example, an epoxy resin. The tube and rods are flexible so that they can be more readily used in narrow advance heading rooms, for example, in caps.
To prevent the liquid resin from flowing into rocks joints, a glass-fiber sheath can be placed over the metallic tubular element, for example a steel tube or at least over the section disposed in the bore hole. This web can, according to the need, be used to wrap the reinforcing elements protruding beyond the steel tube inside the bore hole. The size of the mesh apertures of the glass-fiber web or fabric is such that the plastic material cannot flow therethrough; however, under pressure, that is to say at the end of the injection, the web lets the liquid synthetic resin penetrate through in such a way that it can bind itself to the rock. Thus, the glass-fiber web forms also an additional reinforcement of the anchor.
The initial setting time of the epoxy resin which is generally utilized for the injection under pressure ranges from 6 to 8 minutes, while the complete curing time ranges between 20 and 40 minutes. During this time, the reinforcing means with the help of the anchor plate and the screwing device may be tensioned by an optional tensional force because, on one hand, the reinforcing means is fixed in the anchor pipe head by the cured epoxy resin and on the other hand, the injected epoxy resin first cures at the end of the bore hole, since in this location it will at first be at rest and harden. After this period of hardening, e.g., about 40 minutes, the anchor can be tensioned by the screwing device. Thanks to the sealing rings disposed around the steel pipe which sealingly contact the bore hole wall, a flowing out of the injected resin between the external face of the steel pipe and the internal wall of the bore hole is prevented.
The advantage of the rock anchor according to the present invention is that it permits the pretensioning and stressing of the anchor filled with synthetic resin due to the screw thread, the plate and the clamp nut. Furthermore, it is maintained and sealed in the bore hole by the sealing rings and, finally, the anchor transfers the tensile stress of the tensioning device to the glass-fiber reinforcing means and the filled synthetic resinous materials due to the indentations along the length of the anchor pipe which increase the static friction. Thus, during filling and before hardening, the anchor pipe head together with the inserted reinforcing means and the filled in synthetic resin is better held and fixed in the bore hole also when positioned overhead, and no support is necessary. Because of the indentations of the pipe, the anchor pipe head held without support does not only resist against the injection pressure, but also the static friction between the anchor pipe head and the filling material is increased to such an extent that a high tensile and pretensioning force can be transferred to the reinforcement.